1. Field of the Invention
The present invention relates to a toner for developing electrostatic images for use in electrophotographic apparatuses utilizing the electrophotographic process such as copying machines, printers and facsimiles, and an electrostatic image developer and an image-forming method using the same.
2. Description of the Related Art
Conventionally, various fixing methods for fixing toner for developing electrostatic images (hereinafter, referred to simply as “toner”) are known, including a pressure fixing method using only a pressure roll at room temperature, a contact-heating fixing method using a heating roll or the like, an oven fixing method using oven heating, a flash fixing method using, for example, a xenon lamp, an electromagnetic wave fixing method using, for example, a microwave, and non-contact fixing methods such as a solvent fixing method using solvent vapor. Among them, the oven-heating and contact-heating fixing methods have been predominantly used due to their reliability and stability.
In particular, the contact-heating fixing methods using, for example, a heating roll and a belt, by which images are fixed in a fixing unit normally including a combination of a heating roll or belt provided with a heat source and a pressure roll or belt by bringing the toner image face of a recording medium into contact with, while conveying it past, the heating roll or belt surface under pressure, are higher in thermal efficiency and allow rapid fixing because the heating roll or belt surface and the toner image face of the recording material are brought into contact with each other directly, and thus are used widely.
In these thermal-fixing methods, fixing at a lower temperature is desirable for reducing energy consumption and shortening the so-called warm-up time, i.e., the period from the time of turning on the device to the time when the temperature of the fixing unit reaches operation temperature and the fixing unit becomes ready for operation. Recently, in particular, there has been demand for interrupting the power supply to the fixing unit when it is not in use for further energy conservation, which in turn requires that the fixing member in the fixing unit is heated to a desirable fixing temperature immediately after the start of power supply, and thus fixing at a yet further reduced temperature is desirable.
Reduction of the fixing temperature is also favorable from the viewpoint of cost, as it enables acceleration of printing speed even at the same power consumption level and extension of the lifetime of the heating roll and other parts in the contact-heating fixing method.
However, in conventional methods, a decrease in the fixing temperature of toner is accompanied by a decrease in the glass transition point of the toner particles, which makes it difficult to preserve the storability of the toner. Accordingly, for well-balanced low-temperature fixing and toner storability, it is necessary to provide the toner with a so-called sharp melting property, by which the toner retains a high glass transition point and exhibits a sharp decrease in viscosity in the high temperature range.
However, amorphous resins, which are the resins commonly used for toner, have, for example, glass transition points and molecular weights in relatively wide ranges, and thus, it is necessary to control the composition and molecular weight of a resin extremely precisely to give a resin having a sharp melting property. However, such strict control of a resin demands a special manufacturing process for production of the resin or a special treatment for control of the resin molecular weight (for example, purification of the resin by chromatography), thereby raising the cost for producing the resin and also providing an unnecessary resin, which is not preferable from the recent perspective of environmental protection.
Methods of using a crystalline resin as the binder resin in order to improve low-temperature fixing efficiency have been studied (e.g., Japanese Patent Application Publication (JP-B) Nos. 56-13943, 62-39428 and 63-25335, the disclosures of which are incorporated by reference herein). By using a crystalline resin, low-temperature fixing can be attained, since a toner retains its hardness up until the melting point of the crystal and exhibits a sharp decrease in viscosity at a temperature exceeding the melting point together with the melting of the crystal. However, the crystalline resins described in the literature above were problematic in that the fixing performance thereof on paper is insufficient.
Crystalline resins promising to have improved fixing efficiency on paper are crystalline polyester resins. A method of using a noncrystalline polyester resin having a glass transition temperature of higher than 40° C. and a crystalline polyester resin having a melting point of 130° C. to 200° C. in combination has been proposed as a toner using crystalline polyester resins (e.g., JP-B No. 62-39428, the disclosure of which is incorporated by reference herein). Although the toner prepared by the method is superior in pulverization efficiency and blocking resistance, it is not possible to fix the toner at a temperature lower than before because the melting point of the crystalline polyester resin is relatively high.
Alternatively, toners containing a resin having a melting point of 110° C. or less (as the crystalline resin) and an amorphous resin in combination have also been proposed (e.g., JP-B No. 4-30014, the disclosure of which is incorporated by reference herein). However, combined use of crystalline and amorphous resins leads to a drop in the melting point of the toner, causing problems such as toner blocking and deterioration in powder flowability.
Alternatively, a method of using a crystalline polyester resin and an amorphous resin in combination and fusing the crystalline component using the history of the temperature in the fixing step, and thus preventing blocking of paper during discharge from the fixing unit and also retaining transparency, has been proposed (e.g., Japanese Patent Application Laid-Open (JP-A) No. 2003-50478, the disclosure of which is incorporated by reference herein). However because the crystalline and amorphous resins are fused with each other in the fixed image and thus the melting point of the toner in the fixed image is brought down as described above, the method causes the problem that the toner images once formed on a paper are often transferred onto another paper facing the printed image, resulting in image defects, when the images are exposed to pressure and heat, for example, during conveyance, transportation, or storage in summer.
If the binder resin contains an amorphous resin component in a greater amount, the toner has properties more dependent on the amorphous resin component, and thus it is difficult to lower the fixing temperature beyond previous reductions. Thus, for practical use, it is necessary to use a crystalline resin alone or in combination with an extremely small amount of an amorphous resin. As described above, it has been difficult with the conventional technology to simultaneously satisfy the requirements in low-temperature fixing efficiency, toner storage stability, and storage stability of a fixed image under heat and pressure.